Realistic Rain : In this post i will show you how to add rain to your photographs using noise and motion blur in photoshop.

First lets create a new layer for rain. fill layer with black [ keyboard short shift + backspace ].

Go to filter tab and add noise to layer. again Go to filter and this time add motion blur to layer. keep angel 80 degree and click ok.
adjust layer with levels to make it black and white. Now change rain layer blend to screen

while raining the whole atmosphere looks darker and less saturated. so make darkness and saturation with adjustments. reduce darkness & saturation, increase contrast.
now add gaussian blur to rain layer. duplicate rain layer if you want more rain.
group all layer and name the group rain effect.